Dust filter

ABSTRACT

A dust filter of an evaporated fuel treatment device is taught that preferably include a filtration member for filtering air introduced to an atmosphere introduction path, and a casing receiving the filtration member therein. The dust filter being disposed on the atmosphere introduction path of the evaporated fuel treatment device. The filtration member has an arcuate shape in cross section, and wherein the filtration member is arranged and structured such that air flows in a radial direction thereof, so as to be filtered.

TECHNICAL FIELD

The present invention relates to a dust filter of an evaporated fueltreatment device for vehicles.

BACKGROUND ART

Conventionally, in an evaporated fuel treatment device, a dust filter isused for preventing an adsorbent in a canister from clogging by dust.The dust filter of this type is, for example, disclosed in JapaneseLaid-Open Patent Publication No. 2003-252071.

The dust filter has a filtration member having a cylindrical shape. Thefiltration member is arranged so as to surround an inlet pipe. A dustfilter in an embodiment is arranged and structured such that air flowsthrough the filtration member in an axial direction thereof. Further, adust filter in a modified form is arranged and structured such that airis introduced into the filtration member from an outer circumferentialside in a radial direction and flows downwardly in an axial direction.Further, a dust filter in another modified form is arranged andstructured such that air is introduced into a filtration member from anupper end side in an axial direction thereof and flows inwardly along aradial direction.

However, the above-mentioned dust filter may sometimes have aninsufficient filtration effect. Further, the above-mentioned dust filtermay generally be easily clogged.

Further, an another dust filter is disclosed in Japanese Laid-OpenPatent Publication No. 02-70967. The dust filter has a plate-shapedfiltration member, and the filtration member is arranged so as to closea path of a canister. In the dust filter thus constructed, air flows ina thickness direction of the plate-shaped filtration member.

However, the dust filter can be easily clogged. This may lead toincreased air-flow resistance.

It is, therefore, an object of the present invention to improve aconventional dust filter.

BRIEF SUMMARY OF THE INVENTION

In an embodiment of the present invention, a dust filter of anevaporated fuel treatment device may include a filtration member forfiltering air introduced to an atmosphere introduction path, and acasing receiving the filtration member therein. The dust filter beingdisposed on the atmosphere introduction path of the evaporated fueltreatment device. The filtration member has an arcuate shape in crosssection, and wherein the filtration member is arranged and structuredsuch that air flows in a radial direction thereof, so as to be filtered.

According to the dust filter, air flowing through the atmosphereintroduction line is filtered by passing through the filtration member,so that the dust contained in the air is collected by the filtrationmember.

Also, the filtration member has an arcuate shape in cross section.Therefore, a filtration area of the filtration member can be increasedand decreased by simply increasing and decreasing an axial size of thefiltration member without changing a radial size (i.e., an outerdiameter and/or an inner diameter) thereof.

Further, air flows in the radial direction of the filtration member.Accordingly, the dust collected on the atmosphere introduction side ofthe filtration member can be easily removed by using backflow of air.Therefore, the dust filter cannot be easily clogged.

Further, in another embodiment of the present invention, a dust filtermay include a filtration member for filtering air, a casing receivingthe filtration member, a dust trap chamber communicating with anatmosphere introduction space portion, so as to receive dust removedfrom the filtration member, and a backflow prevention device preventingbackflow of the dust from the dust trap chamber to the atmosphereintroduction space portion.

According to the dust filter, the backflow of the dust received in thedust trap chamber via the atmosphere introduction space portion isprevented by the backflow prevention wall. Therefore, the dust removedfrom the filtration member can be effectively prevented from depositingin the atmosphere introduction space portion. As a result, the air-flowresistance of the filtration member 44 can be effectively prevented fromrising.

Other objects, features, and advantages, of the present invention willbe readily understood after reading the following detailed descriptiontogether with the accompanying drawings and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of an evaporated fuel treatment device inwhich a dust filter according to a first embodiment of the presentinvention is used;

FIG. 2 is a sectional view of the dust filter;

FIG. 3 is a sectional view taken along the line III-III of FIG. 2;

FIG. 4 is a sectional view of the dust filter according to a secondembodiment of the present invention; and

FIG. 5 is a sectional view taken along the line V-V of FIG. 4.

DETAILED DESCRIPTION OF THE INVENTION

Representative embodiments of the present invention will now bedescribed with reference to the drawings.

First Embodiment

First, a first embodiment of the present invention will be describedwith reference to FIGS. 1 to 3.

As shown in FIG. 1, in an evaporated fuel treatment device 10, anevaporated fuel is adsorbed by an adsorbent 22 in a canister 20 disposedin a middle of a purge line 18 that communicates between a fuel tank 12and an inlet pipe 16 of an engine 14 (an internal combustion engine).Based on a driving state of the engine 14, a purge valve 24 disposed inthe purge line 18 is opened and closed. As a result, the evaporated fueladsorbed by the adsorbent 22 in the canister 20 is introduced into theinlet pipe 16 through the purge line 18.

The fuel tank 12 is provided with an inlet pipe 30 having a fill opening31. The inlet pipe 30 is a pipe for introducing a fuel through the fillopening 31 into the fuel tank 12. The fill opening 31 is openedobliquely upwardly. A cap 33 is detachably attached to the fill opening31. Further, the inlet pipe 30 and the fuel tank 12 are communicatedwith each other via a breather line 35.

In the canister 20, the evaporated fuel generated in the fuel tank 12 isadsorbed by the adsorbent 22 made of activated carbon or the like, so asto be from being released to an atmosphere through an atmosphereintroduction line 26. The evaporated fuel adsorbed by the canister 20 isintroduced into the inlet pipe 16 through the purge line 18 with the aidof a negative pressure in the inlet pipe 16 when the purge valve 24 isin an open state, so as to be burned in the engine 14. At this time, airis introduced into the inlet pipe 16 through the atmosphere introductionline 26. Further, the atmosphere introduction line 26 introduces airfrom the atmosphere into the fuel tank 12 or discharges air from thefuel tank 12 to the atmosphere, so that pressure fluctuation produced inthe fuel tank 12 can be reduced. Further, the pressure fluctuationproduced in the fuel tank 12 may involve a negative pressure that iscaused by fuel consumption, temperature reduction, purging or the like,and a positive pressure that is caused by the evaporated fuel due totemperature rise, fuel vibration or the like. Further, in FIG. 1,reference numerals 37, 38 and 39 respectively correspond to a throttlevalve, an air cleaner and an air cleaner element.

Further, a dust filter 40 is attached to the fill opening 31 of theinlet pipe 30. The dust filter 40 has a shape that surrounds at least apart of a circumference of the fill opening 31. The dust filter 40 ofthis embodiment communicates with an atmosphere side end portion of theatmosphere introduction line 26, which end portion is positioned at acircumferential portion of the fill opening 31 of the inlet pipe 30. Thedust filter 40 may filter air introduced into the atmosphereintroduction line 26, thereby removing dust contained therein. Further,directions (forward, rearward, leftward and rightward) shown in thedrawings should not be construed restrictively.

As shown in FIGS. 2 and 3, the dust filter 40 may include a casing 42and a filtering member 44.

The casing 42 is made of, for example, a resin, and has asemicylindrical shape that can surround a lower half of the fill opening31 of the inlet pipe 30 (see FIG. 1). The casing 42 may have asemicylindrical inner recessed portion which is coupled to the fillopening 31. The casing 42 includes an inner circumferential wall portion46 having a semicircular shape in cross section, an outercircumferential wall portion 47 having a semicircular shape in crosssection, circumferential end wall portions 48 and 49 (see FIG. 2)connecting circumferential end portions of the inner circumferentialwall portion 46 and the outer circumferential wall portion 47, and upperand lower end plate portions 50 and 51 (see FIG. 3) having an arcuateshape and connecting axial end portions (end portions in a depthdirection of a plane of FIG. 2) of the inner and outer circumferentialwall portions 46 and 47 and the circumferential end wall portions 48 and49. Further, the outer circumferential wall portion 47 is positioned inparallel to the inner circumferential wall portion 46 at a predeterminedinterval therefrom. The casing 42 thus constructed may have a sealedinner space having the semicylindrical shape.

As shown in FIG. 3, the casing 42 has a partition wall portion 52positioned in parallel to the lower end plate portion 51 at apredetermined interval, so that the inner space thereof is divided to anupper filtration chamber 54 and a lower dust trap chamber 55. Thus, thecasing 42 has a structure in which a plurality of (two in thisembodiment) divided bodies are joined to each other. That is, the casing42 is structured by arranging a filtration member 44 in the filtrationchamber 54 (one of the divided bodies) and joining the divided bodies toeach other. Further, the casing 42 is fixed to the fill opening 31 ofthe inlet pipe 30 by using appropriate fasteners such as screws andclips (not shown). Further, since the fill opening 31 is generallyopened obliquely upwardly, the casing 42 is also positioned so as to beinclined along the fill opening 31 (see FIG. 3). Therefore, when thedust filter 40 is assembled to a vehicle, as shown in FIG. 3, thefiltration chamber 54 is upwardly positioned and the dust trap chamber55 is downwardly positioned. Further, the filtration member 44 (whichwill be described hereinafter) is positioned such that an innercircumferential side (an upper side in FIGS. 2 and 3) thereof isdirected upwardly and that an outer circumferential side (a lower sidein FIGS. 2 and 3) thereof is directed to downwardly.

As shown in FIG. 2, a first support piece 57 is formed in the innercircumferential wall portion 46 at one end side (a left end side in FIG.2) in a circumferential direction thereof. The first support piece 57projects outwardly in a radial direction and extending in an axialdirection. A distal end portion of the first support piece 57 extends toa middle portion in the radial direction of the filtration chamber 54.Conversely, a second support piece 58 is formed in the outercircumferential wall portion 47 at one end side (a right end side inFIG. 2) in a circumferential direction thereof. The second support piece58 projects inwardly in the radial direction and extending in an axialdirection. A distal end portion of the second support piece 58 extendsto a middle portion in the radial direction of the filtration chamber54.

The filtration member 44 is formed by bending filtration materials in apleat-shaped manner. Examples of the filtration materials may includefiltration paper, non-woven fabric and air-permeable foamed urethane.The filtration member 44 is received in the filtration chamber 54. Oneend portion (a left end portion in FIG. 2) 44 a in the circumferentialdirection of the filtration member 44 is coupled or connected to thefirst support piece 57. Further, the other end portion (a right endportion in FIG. 2) 44 b in the circumferential direction of thefiltration member 44 is coupled or connected to the second support piece58. Further, an upper edge portion of the filtration member 44 isconnected to the upper end plate portion 50, and a lower edge portion ofthe filtration member 44 is connected to the partition wall portion 52(see FIG. 3). As a result, the filtration chamber 54 is divided into twospace portions by the filtration member 44. One of the two spaceportions is positioned adjacent to the outer circumferential side (anatmosphere introduction side) of the filtration member 44. The spaceportion thus positioned is referred to as an atmosphere introductionspace portion 60 (see FIGS. 2 and 3). Conversely, the other of the twospace portions is positioned adjacent to the inner circumferential side(an atmosphere emission side) of the filtration member 44. The spaceportion thus positioned is referred to as an atmosphere emission spaceportion 62 (see FIGS. 2 and 3).

An atmosphere releasing opening 64 is formed in the casing 42 (see FIG.2). The atmosphere releasing opening 64 is communicated with theatmosphere introduction space portion 60, so as to open the atmosphereintroduction space portion 60 to an atmosphere. With this structure, airin the atmosphere can be introduced into the atmosphere introductionspace portion 60 of the filtration chamber 54 through the atmospherereleasing opening 64 (see an arrow Y1 in FIG. 2). As will beappreciated, the position of the atmosphere releasing opening 64 is notlimited to the position shown FIG. 2.

An air emission opening 66 is formed in the casing 42 (see FIG. 2). Theair emission opening 66 is communicated with the atmosphere emissionspace portion 62, so as to open the atmosphere emission space portion 62(see FIG. 2). The air emission opening 66 communicates with anatmosphere-side end portion of the atmosphere introduction line 26 (seeFIG. 1). With this structure, air in the atmosphere emission spaceportion 62 of the filtration chamber 54 can be emitted into theatmosphere introduction line 26 (see FIG. 1) through the air emissionopening 66, and then be introduced to the canister 20. As will beappreciated, the position of the air emission opening 64 is not limitedto the position shown FIG. 2.

As shown in FIG. 3, the partition wall portion 52 of the casing 42 isformed with a communication opening (a dust trap chamber inlet port) 68that communicate between the atmosphere introduction space portion 60 ofthe filtration chamber 54 and the dust trap chamber 55. Thecommunication opening 68 is positioned so as to be adjacent to an innercircumferential surface of the outer circumferential wall portion 47.Further, the communication opening 68 is formed in at least the vicinityof a lowest portion in a circumferential direction of the outercircumferential wall portion 47. That is, the communication opening 68is substantially formed in the lowermost portion of the partition wallportion 52. The communication opening 68 may be formed as a singleopening or a plurality of openings. Further, the communication opening68 may be referred to as “an inlet portion of the dust trap chamber.”

A rib-shaped backflow prevention wall (a backflow prevention device) 70is formed in the outer circumferential wall portion 47, so as to coverthe communication opening 68 in a weir-like manner. The backflowprevention wall 70 is positioned adjacent to the lowermost portion ofthe partition wall portion 52. Further, the backflow prevention wall 70projects inwardly in the radial direction and extending in thecircumferential direction. Further, as shown in FIG. 3, the backflowprevention wall 70 has a height greater than the height of thecommunication opening 68. Also, the backflow prevention wall 70 isinclined relative to the partition wall portion 52, so that a distal endthereof is gradually spaced away from the the partition wall portion 52.

In the dust filter 40, air flows into the atmosphere introduction spaceportion 60 (see the arrow Y1 in FIG. 2) through the atmosphere releasingopening 64 (see FIG. 2). The air radially flows through the filtrationmember 44 from the outer circumferential side toward the innercircumferential side, so as to be filtered. The filtered air flows intothe atmosphere emission space portion 62 (see an arrow Y2 in FIGS. 2 and3). Further, the filtered air flows out through the air emission opening66 (see an arrow Y3 in FIG. 2). The air is introduced into the canister20 through the atmosphere introduction line 26, as shown in FIG. 1.

Further, dust is collected on an outer circumferential side of thefiltration member 44 facing the atmosphere introduction space portion60. When the dust filter 40 is assembled to the vehicle, the outercircumferential side of the filtration member 44 is directed downwardly.Accordingly, the dust collected on the outer circumferential side of thefiltration member 44 may preferably be removed therefrom due to agravity, vehicle vibration, a positive pressure at a time of feeding oilor the like and falls on an inner circumferential surface of the outercircumferential wall portion 47 (see arrows Y4 in FIG. 3). Further, thedust are introduced into the dust trap chamber 55 via the communicationopening 68 and are received therein (see an arrow Y5 in FIG. 3). At thistime, the dust is introduced into a lower space portion of the dust trapchamber 55 beyond the backflow prevention wall 70. Therefore, backflowof the received dust (dust D) can be effectively prevented or reduced bythe backflow prevention wall 70.

Further, the dust filter 40 is positioned on an atmosphere introductionpath of the canister 20 (the atmosphere-side end portion of theatmosphere introduction line 26 in this embodiment) (see FIG. 1).Therefore, the dust is collected by the filtration member 44 (see FIGS.2 and 3) when a negative pressure is produced in the canister 20. Also,when a positive pressure is produced in the canister 20, the dust isremoved from the filtration member 44 by the pressure (see arrows Y4 inFIG. 3). Therefore, collection of the dust by the filtration member 44and removal of the dust from the filtration member 44 can be effectivelyperformed. Further, “the atmosphere introduction path” may include anair flow path in the dust filter 40 and the atmosphere introduction line26.

According to the dust filter 40 of the evaporated fuel treatment device10, air flowing through the atmosphere introduction line 26 is filteredby passing through the filtration member 44, so that the dust containedin the air is collected by the filtration member 44.

Also, the filtration member 44 has an arcuate shape in cross section.Therefore, unlike the conventional dust filter shown in, for example,Japanese Laid-Open Patent Publication No. 2003-252071, a filtration areaof the filtration member 44 can be increased and decreased by simplyincreasing and decreasing an axial size of the filtration member 44without changing a radial size (i.e., an outer diameter and/or an innerdiameter) thereof.

Further, in this embodiment, air flows in the radial direction of thefiltration member 44. Accordingly, the dust collected on onecircumferential side (the atmosphere introduction side) of thefiltration member 44 can be easily removed by using backflow of air.Therefore, compared with the conventional dust filter, a removal effectof the dust by the backflow of air can be increased.

The casing 42 may surround the lower half of the fill opening 31 of theinlet pipe 30 of the fuel tank 12 (see FIGS. 2 and 3). Therefore, thecasing 42 can cushion the fill opening 31 of the inlet pipe 30 at a timeof vehicle collision. Further, the casing 42 can be modified to havecylindrical shape, so as to substantially circumferentially surround thefill opening 31 of the inlet pipe 30. The casing 42 thus modified mayfurther effectively protect the fill opening 31 of the inlet pipe 30 atthe time of vehicle collision. Further, the casing 42 may be modified soas to have, for example, a C-shape, a rectangular cylindrical shape, aD-shape or other such shapes.

Further, in this embodiment, when the dust filter 40 is assembled to thevehicle, the atmosphere introduction side of the filtration member 44corresponds to the outer circumferential side of the filtration member44 that is directed downwardly. Accordingly, the dust is collected onthe outer circumferential side of the filtration member 44. Thecollected dust may preferably be removed therefrom due to a gravity,vehicle vibration, a positive pressure at a time of feeding oil or thelike. Therefore, air-flow resistance of the filtration member 44 can beeffectively prevented from rising.

In this embodiment, the filtration member 44 has the arcuate shape incross section, so that the air can radially flows from the outercircumferential side of the filtration member 44 toward the innercircumferential side thereof (see the arrow Y2 in FIGS. 2 and 3). Thatis, the outer circumferential side of the filtration member 44 havingthe arcuate shape may function as a dust collecting surface. Therefore,the area of the dust collecting surface of the filtration member 44 maypreferably be increased.

Further, the dust removed from the filtration member 44 is received inthe dust trap chamber 55 via the atmosphere introduction space portion60 (see FIG. 3). The backflow of the dust received in the dust trapchamber 55 is prevented or reduced by the backflow prevention wall 70.Therefore, the dust removed from the filtration member 44 can beeffectively prevented from depositing in the atmosphere introductionspace portion 60. As a result, the air-flow resistance of the filtrationmember 44 can be effectively prevented from rising.

Further, the backflow prevention wall 70 is a rib-shapedsimply-structured member that can cover the communication opening 68 ina weir-like manner. Therefore, the dust filter 40 can be simplified.

Second Embodiment

The second detailed representative embodiment will now described withreference to FIGS. 4 and 5.

Because the second embodiment relates to the first embodiment, only theconstructions and elements that are different from the first embodimentwill be explained in detail. Elements that are the same in the first andsecond embodiments will be identified by the same reference numerals anda detailed description of such elements may be omitted.

In this embodiment, an additional filtration member 43 is attached tothe outer circumferential side (i.e., the side for collecting the dust)of the filtration member 44 of the dust filter 40. In this embodiment,the filtration members 43 and 44 will be referred to as “a filtrationmember 45.” In addition, the additional filtration member 43 and thefiltration membr 44 are respectively referred to as “an upstreamfiltration portion 43” and “a downstream filtration portion 44.”

The upstream filtration portion 43 is made of sheet-shaped filtrationmaterials. Examples of the filtration materials may include filtrationpaper, non-woven fabric and air-permeable foamed urethane. The upstreamfiltration portion 43 is laminated on and connected to the outercircumferential side of the downstream filtration portion 44. Thedownstream filtration portion 44 is a main filtration portion and has amesh size that can appropriately collect the dust contained in airpassing through the casing 42. Conversely, the upstream filtrationportion 43 is a subsidiary filtration portion. The upstream filtrationportion 43 has a mesh size greater than the mesh size of the downstreamfiltration portion 44, so as to collect only the dust having arelatively large size contained in the air passing through the casing42.

In the dust filter 40, the air flowing into the atmosphere introductionspace portion 60 (see the arrow Y1 in FIG. 4) radially passes throughthe upstream filtration portion 43 and the downstream filtration portion44 from the outer circumferential side toward the inner circumferentialside, so as to be filtered. The filtered air flows into the atmosphereemission space portion 62 (see the arrow Y2 in FIG. 4). At this time,the dust having the relatively large size in the air is collected by theupstream filtration portion 43. The remaining dust (i.e., the dusthaving a relatively small size) in the air is collected by thedownstream filtration portion 44.

Further, the dust is collected on an outer circumferential side of theupstream filtration portion 43 facing the atmosphere introduction spaceportion 60. When the dust filter 40 is assembled to the vehicle, theouter circumferential side of the upstream filtration portion 43 isdirected downwardly. Accordingly, the dust collected on the outercircumferential side of the upstream filtration portion 43 maypreferably be removed therefrom due to a gravity, vehicle vibration, apositive pressure at a time of feeding oil or the like and falls on theinner circumferential surface of the outer circumferential wall portion47 (see the arrows Y4 in FIG. 5). Further, the dust are introduced intothe dust trap chamber 55 via the communication opening 68 and arereceived therein (see the arrow Y5 in FIG. 5). At this time, the dust isintroduced into the lower space portion of the dust trap chamber 55beyond the backflow prevention wall 70. Therefore, backflow of thereceived dust (a dust D) can be effectively prevented or reduced by thebackflow prevention wall 70.

The dust filter 40 of the second embodiment may have substantially thesame function and effects as the first embodiment.

However, unlike the first embodiment, the filtration portions 43 and 44of the filtration member 45 respectively have different mesh sizes. Thatis, the upstream filtration portion 43 has the mesh size greater thanthe mesh size of the downstream filtration portion 44. In other words,the filtration member 45 has a gradient in mesh size. Therefore, thedust can be separately collected based on the size thereof. As a result,the filtration member 45 may have an improved filtration performance. Inaddition, the air-flow resistance of the filtration member 45 can befurther effectively prevented from rising.

Naturally, various changes and modifications may be made to the presentinvention without departing from the scope of the invention. Forexample, the dust filter 40 of the present invention can be applied as afilter of a general air cleaner or the like for filtering air (includingother gases). Further, the dust filter 40 of this embodiment can bepositioned on any portion on the atmosphere introduction path.

Further, in the second embodiment, the upstream filtration portion 43and the downstream filtration portion 44 of the filtration member 45 canbe made of the different filtration materials or the same filtrationmaterial. Further, although the filtration member 45 includes the twofiltration portions 43 and 44, the filtration member 45 may includethree or more filtration portions that respectively have different meshsizes. Conversely, the filtration member 45 may includes only afiltration portion in which mesh size thereof is continuously changedfrom the atmosphere introduction side to the atmosphere emission side.In addition, each of the two filtration portions 43 and 44 may includetwo or more layered portions. Also, the upstream filtration portion 43and the downstream filtration portion 44 can be appropriatelypositioned, so as to be spaced apart from each other.

Further, the backflow prevention wall 70 may be a separately formedmember that is movably attached to the outer circumferential wallportion 47, so as to be opened and closed by using an elastic material,a spring material or the like. Further, the backflow prevention wall 70can be replaced with a check valve such as a flap valve. Further, in theembodiments, although the dust filter 40 is designed such that the outercircumferential side of the filtration member 44 or the filtrationmember 45 corresponds to the atmosphere introduction side thereof, thedust filter 40 can be modified such that the inner circumferential sideof the filtration member 44 or the filtration member 45 corresponds tothe atmosphere introduction side thereof, if necessary. Also, theelements such as the casing 42, the filtration members 44 and 45, thedust trap chamber 55 and the backflow prevention wall 70 may havevarious shapes.

Representative examples of the present invention have been described indetail with reference to the attached drawings. This detaileddescription is merely intended to teach a person of skill in the artfurther details for practicing preferred aspects of the presentinvention and is not intended to limit the scope of the invention. Onlythe claims define the scope of the claimed invention. Therefore,combinations of features and steps disclosed in the foregoing detaildescription may not be necessary to practice the invention in thebroadest sense, and are instead taught merely to particularly describedetailed representative examples of the invention. Moreover, the variousfeatures taught in this specification may be combined in ways that arenot specifically enumerated in order to obtain additional usefulembodiments of the present invention.

1. A dust filter of an evaporated fuel treatment device, the dust filterbeing disposed on an atmosphere introduction path of the evaporated fueltreatment device, comprising: a filtration member for filtering airintroduced to the atmosphere introduction path; and a casing receivingthe filtration member therein, wherein the filtration member has anarcuate shape in cross section, and wherein the filtration member isarranged and structured such that air flows in a radial directionthereof, so as to be filtered.
 2. The dust filter as defined in claim 1,wherein the filtration member has a gradient in mesh size, so that anupstream side thereof has a mesh size greater than the mesh size of adownstream side thereof.
 3. The dust filter as defined in claim 1,wherein the casing surrounds at least a part of a circumference of afill opening of an inlet pipe of a fuel tank.
 4. The dust filter asdefined in claim 1, wherein the filtration member is positioned suchthat a lower side thereof corresponds to an atmosphere introductionside.
 5. A dust filter, comprising: a filtration member for filteringair; a casing receiving the filtration member; a dust trap chambercommunicating with an atmosphere introduction space portion, so as toreceive dust removed from the filtration member; and a backflowprevention device preventing backflow of the dust from the dust trapchamber to the atmosphere introduction space portion.
 6. The dust filteras defined in claim 5, wherein the backflow prevention device comprisesa backflow prevention wall that is arranged and constructed to cover adust trap chamber inlet port in a weir-like manner.
 7. The dust filteras defined in claim 5, wherein the dust filter is disposed on anatmosphere introduction path of an evaporated fuel treatment device. 8.The dust filter as defined in claim 5, wherein the casing surrounds atleast a part of a circumference of a fill opening of an inlet pipe of afuel tank.
 9. The dust filter as defined in claim 8, wherein thefiltration member is arranged and structured such that air flows in aradial direction thereof, so as to be filtered